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S = 10 SGE (S Sagittae = 10 Sagittae). Among the most importantkinds of stars in the sky are theCepheid variables, named afterthe prototypeDelta Cephei. Mid-temperature evolvingsupergiants, they have losttheir sense of stability. All fall within the famed "instabilitystrip" that runs down the middle of theHRdiagram (a plot of absolute magnitude against spectral class). Expanding and contracting, Cepheids typically vary by one or moremagnitudes over a period of a few days.EtaAquilae andMekbuda (Zeta Geminorum)are among their ranks, as isPolaris(which has uniquely almost stopped varying). In 1912, HenriettaLeavitt of the Harvard College Observatory discovered that theCepheids' absolute brightnesses are intimately tied to theiroscillation periods. Measure a Cepheid's period, get theluminosity, compare it to the apparent brightness, and out fallsthe distance. Cepheids thus become "standard candles" that areused to find the distances of othergalaxies and that are cruciallinks in measuring theexpansion rate and age of theUniverse. Their discovery and measure in other galaxies was amongthe "key projects" for the Hubble Space Telescope. They were firstused by Edwin Hubble to establish the existence of externalgalaxies. Seventeen are of magnitude 6.0 or brighter, hence aremore or less visible to the naked eye (25 of them brighter than6.5), includingX andW Sagittarii. Eight degrees almost duenorth ofAltair inAquila lies another, S Sagittae, inSagitta, the Arrow, the star also known by itsFlamsteed number, 10 Sge. Nominally a class G (G5) supergiant, the star varies between fifthand sixth magnitudes (5.3 and 6.1) over a period of 8.382 days, theclass running as early as F5. S Sge is so far away thatparallax is pretty useless,giving us 3200 light years with an uncertainty of a couplethousand. It's far better to use the standard relation betweenperiod and luminosity, which results in an absolute magnitude (whatthe apparent magnitude would be at a distance of 32.6 light years)of minus 4.0. Allowing for a few tenths of a magnitude of dimmingbyinterstellar dust, comparisonwith the apparent magnitude then gives a much more certain distanceof 2300 light years. With a temperature averaging around 5500Kelvin, S Sagittae glows with a luminosity (which changes with thecycle) of around 3500 times that of theSun. Temperature and luminosity then conspire to yield a radius of 66times solar, 0.3 Astronomical Units, which approaches the size ofMercury's orbit. Theory gives a mass around 6 or 7 times that ofthe Sun and an age of at least 43 million years. Not massiveenough to explode as asupernova, S Sge will die asa fairly heftywhitedwarf. As a bit of an anticlimax, the star has a lessercompanion as found throughDoppler shifts in itsspectrum. With a period of 1.85years, at low mass the neighbor would orbit at an average distanceof 2.9 Astronomical Units, a safe distance at which to watch theCepheid show proceed.
Written byJim Kaler 10/04/13. Return toSTARS.
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